1. Field of the Invention
The present invention relates to keyboards and capacitive keys for use in such keyboards, and particularly relates to an improved mechanical design in which overtravel is provided by a novel spring means integral with the movable capacitor plate.
Keyboards such as that to which the present invention relates are widely used in information processing input terminals. U.S. Pat. No. 3,750,113 to Cencel, assigned to the assignee of the present invention, discloses the circuit techniques for utilizing capacitive keys of the type described in the present invention. The present application is directed mainly to the mechanical implementation of such keys. Each key corresponds to a particular piece of data or operation which the operator selects by depressing the key. Typically, a modest preload is applied to the key to maintain it in its normal position, thereby reducing the likelihood of accidental actuation and requiring a positive, conscious effort by the operator to depress the key. Typically, the key surface touched by the operator is connected to a plunger or lever which extends into the data processing machine for actuating whatever mechanisms are employed therein.
It is well known in the art to provide for "overtravel" of the plunger. By overtravel is meant that the key and plunger may be depressed beyond the minimum amount required to activate the data processing equipment. Such overtravel results in a number of advantages. First, it assures positive action, because the operator normally will depress the key as far as possible, which means that the key will normally be depressed well beyond the amount necessary to actuate the apparatus. Second, the use of overtravel provides that the plunger will be depressed beyond the minimum required amount for a modest period of time, and this is beneficial in certain types of apparatus for discriminating against noise or inadvertent use of the key. Further, the use of overtravel acts as a shock absorber, tending to reduce bounce that might otherwise occur when the plunger, in being depressed, meets the mechanical resistance of the mechanism being actuated.
2. The Prior Art
In German Pat. No. 1,940,554, filed 8 Aug. 1969, and issued 18 Feb. 1971, Borisow, et al. show the use of a compressible layer attached to the end of the plunger and serving at the same time to insulate the capacitor plate from the plunger. The capacitor plate is attached over the compressible layer. It appears that the main use of the layer is to absord shock, provide electrical insulation and provide overtravel.
In that German patent, the main invention appears to be the conversion of mechanical movement into an electrical signal by means of a capacitor having a plate attached to the plunger and insulated from it and further having at least two other plates fixed on an insulative layer attached to the housing. Typically, in such a three-plate capacitive key, one of the smaller plates, the excitor, is excited with an a.c. signal. The signal is recieved on the other smaller plate, called the receptor. The larger movable plate is left ungrounded (floating). One problem which has been encountered with such devices is that the floating movable plate picks up electrostatic noise and injects it into the system.
This arrangement of a movable capacitor plate opposed by two stationary plates is equivalent to two capacitors in series. The capacitance of each of the capacitors is, of course, directly proportional to the area of one of the stationary plates and inversely proportional to the separation between the plates. Thus, a simple analysis shows that the total capacitance from one of the stationary plates to the other stationary plate, neglecting the sideward interaction between those plates, is only one-fourth the value that could be achieved between the moving plate and a fixed plate of the same area. For this reason, the change in capacitance caused by a change in separation of the plate would be four times greater if the capacitor were formed by a single moving plate and a single fixed plate having the same area. Thus, the three plate capacitor does not make the most efficient use of the available space and motion.
For a least a year prior to the filing of the present application, the assignee of that application has marketed a capacitive key in which a movable capacitor plate is opposed by a stationary plate of substantially equal area. In that product, the movable capacitor plate is affixed to an insulative member having on its opposite side, perpendicular to the plate at its center, a tubular extension held captive to the plunger by a pin passing through the tube, permitting relative motion between the insulative member and the plunger. This motion comprises the overtravel, and it is yieldingly opposed by a spring wrapped about the pin urging the insulative member away from the plunger. As will be seen below, Applicant's present invention achieves comparable results with the use of fewer parts and further, is less complex to assemble.